1. Field of the Invention
The present invention relates to automatic gain control used in radio communication mobile stations and, more particularly, to an automatic gain control method for realizing stable reception characteristics against a change in fading period.
2. Description of the Related Art
Generally, automatic gain control (AGC) used in radio communication mobile stations is performed so as to keep constant the amplitude of an in-phase (I) component signal and that of a quadrature (Q) component signal supplied from a radio unit to a baseband signal processing unit in a radio communication mobile station. FIG. 5 is a block diagram of an example of the arrangement of components for AGC in a conventional radio communication mobile station.
Referring to FIG. 5, the radio communication mobile station includes an antenna 501 for receiving a signal transmitted from a base station, a receiver 502 for radio signal processing, e.g., frequency-converting the received signal having a radio frequency bandwidth into a signal having an intermediate frequency bandwidth, an AGC amplifier 503, of which gain is controlled depending on the power level of the received signal, an orthogonal demodulator 504 for performing orthogonal demodulation and frequency conversion to the intermediate frequency signal to produce I-component and Q-component baseband signals, analog-to-digital (AD) converters 505 for converting the baseband signals, that are analog, into digital signals, respectively, a baseband signal processor 506, an instantaneous received power calculator 507 for calculating the instantaneous power of the received signal from the I-component and Q-component signals, an average received power calculator 508 for calculating the average of the received power, and a feedback AGC code generator 509 for comparing the average received power with reference power, calculating the amount of gain control of the AGC amplifier 503 so as to offset the difference between the average received power and the reference power, and then feeding back the amount of gain control to the AGC amplifier 503.
The conventional radio communication mobile station of FIG. 5 operates as follows.
The antenna 501 receives a signal transmitted from a base station (not shown). The receiver 502 performs radio signal processing, e.g., frequency-converts the received signal having a radio frequency bandwidth into an intermediate frequency signal. The converted signal is supplied to the AGC amplifier 503 and is then supplied to the orthogonal demodulator 504.
The orthogonal demodulator 504 performs orthogonal demodulation-and frequency conversion to the supplied intermediate frequency signal to generate I-component and Q-component baseband signals. The AD converters 505 convert the I-component and Q-component baseband signals that are analog into digital signals, respectively. The baseband signal processor 506 performs digital signal processing, e.g., error correction to the supplied digital signals.
The I-component and Q-component baseband signals generated from the AD converters 505 are also supplied to the instantaneous received power calculator 507. The instantaneous received power calculator 507 calculates the instantaneous power of the received signal from the supplied I-component and Q-component baseband signals. The average received power calculator 508 calculates average received power per time previously set in the radio communication mobile station. The feedback AGC code generator 509 compares the calculated average received power with the reference power held in the radio communication mobile station.
The feedback AGC code generator 509 calculates the amount of gain control of the AGC amplifier 503 so as to offset the difference between the reference power and the average received power calculated by the average received power calculator 508 and then feeds back the amount of gain control as the amount of AGC to the AGC amplifier 503. The gain of the AGC amplifier 503 is controlled according to the amount of AGC fed back.
For example, when the average received power calculated by the average received power calculator 508 is lower than the reference power, the amount of AGC is determined so that the gain of the AGC amplifier 503 is increased to raise the received power to the extent of offsetting the difference between the reference power and the average received power. The determined amount of AGC is fed back to the AGC amplifier 503. The gain of the AGC amplifier 503 is increased depending on the amount of AGC fed back so that the average power of the I-component and Q-component signals to be generated equals the reference power.
On the other hand, when the average received power calculated by the average received power calculator 508 is higher than the reference power, the gain of the AGC amplifier 503 is reduced so that the average received power of the I-component and Q-component signals equals the reference power. According to the above operation, the I-component and Q-component signals supplied to the baseband signal processor 506 are made substantially constant.
In the conventional radio communication mobile station, however, under circumstances where the power of a received signal varies due to fast fading, the gain control of the AGC amplifier 503 cannot follow a steep change in received signal power, resulting in variations of the respective amplitudes of I-component and Q-component signals to be generated. Disadvantageously, an error occurs in the received signal in the baseband signal processor 506. This problem will now be described with reference to FIGS. 6A and 6B in terms of a variation in received signal power caused by fast fading.
FIG. 6A shows a change in power of a received signal and a change in power of the I-component and Q-component signals supplied to the baseband signal processor 506 when a fading pitch is low, i.e., in a slow fading environment. Referring to FIG. 6A, when the fading pitch is low, the gain control of the AGC amplifier 503 can follow a change in received signal power. Obviously, the power of the I-component and Q-component signals supplied to the baseband signal processor 506 is made substantially constant.
FIG. 6B shows a change in power of a received signal and a change in power of the I-component and Q-component signals supplied to the baseband signal processor 506 when a fading pitch is high, i.e., in a fast fading environment. Referring to FIG. 6B, when the fading pitch is high, the gain control of the AGC amplifier 503 cannot follow a steep change in received signal power and the power of the I-component and Q-component signals supplied to the baseband signal processor 506 is fluctuated.
In the conventional radio communication mobile station, the amount of AGC is determined based on the difference between the reference power and the average received signal power per predetermined time. Even when the fading pitch becomes higher, the gain of the AGC amplifier 503 is controlled every predetermined period. Consequently, the gain control of the AGC amplifier 503 cannot follow a steep change in received signal power, resulting in a fluctuation in power of the I-component and Q-component baseband signals.
Japanese Unexamined Patent Application Publication Nos. 2000-59158 (Patent Document 1) and 2000-269759 (Patent Document 2) disclose technologies of generating an automatic gain control signal corresponding to a fading period to prevent a steep change in received signal power depending on fading.
Patent Document 1 discloses a system including detection means for detecting an output signal obtained by amplifying an input signal through a variable gain amplifier, fading period detection means for detecting a fading period indicative of a changing period of the input signal, difference generation means for generating the difference between a target convergence value and a digital value of the voltage of the detected signal, variable multiplication means for variably multiplying the difference depending on the fading period, difference change calculation means for calculating a change in the difference subjected to variable multiplication, and control signal generation means for converting the change in the difference according to a predetermined conversion method to generate an automatic gain control signal for the variable gain amplifier. Thus, the loop gain characteristic can be optimized against a steep change in received signal power depending on fading.
Patent Document 1 further discloses a system including detection means for detecting an output signal obtained by amplifying an input signal through a variable gain amplifier, fading period detection means for detecting a fading period indicative of a changing period of the input signal, difference generation means for generating the difference between a target convergence value and a digital value of the voltage of the detected signal, difference change calculation means for calculating a change in the difference, conversion-method selection means for selecting a conversion method of converting the change in the difference from among a plurality of conversion methods according to the fading period, and means for converting the change in the difference according to the selected conversion method to generate an automatic gain control signal for the variable gain amplifier. The same advantages as those of the above system are obtained.
Patent Document 2 discloses a circuit including control signal generation means for detecting the level of an output signal obtained by amplifying a received signal through a variable gain amplifier to generate a feedback signal as a control signal for the variable gain amplifier, fading pitch detection means for detecting the fading pitch of the received signal, and means for determining generating time or period of the control signal according to the detected fading pitch. Thus, the same advantages as those of the above-mentioned systems are obtained.
According to the invention disclosed in Patent Document 1, the system requires the variable multiplication means for multiplying the difference between the target convergence value and the digital value of the detected voltage depending on the fading period, the difference change calculation means for obtaining a change in the difference subjected to variable multiplication, and the control signal generation means for converting the change in the difference according to the predetermined conversion method to generate an automatic control signal. Unfortunately, the structure of the system is complicated.
According to the invention disclosed in Patent Document 2, data detected by a level detector is always averaged every predetermined time by an averaging unit independent of a fading period. A signal generated from the averaging unit does not follow the fading period. Disadvantageously, the optimum automatic gain control loop cannot be set with respect to various fading periods.
Generally, in controlling an AGC amplifier used in a radio communication mobile station, a received signal having a radio frequency bandwidth is subjected to orthogonal demodulation into I-component and Q-component baseband signals. Average received power per predetermined time is calculated from the I-component and Q-component baseband signals. The calculated average received power is compared to reference power set in the radio communication mobile station to obtain the difference therebetween. The gain of the AGC amplifier is controlled so as to offset the difference. The respective amplitudes of the I-component and Q-component baseband signals are controlled to be always made constant independent of a change in received signal power at an antenna.
So long as the gain control of the AGC amplifier is properly performed, the respective amplitudes of the I-component and Q-component signals obtained by orthogonal demodulation are always kept constant. However, in a case where the fading pitch of a signal received at the antenna is high because the corresponding radio communication mobile station moves at a high speed, when the gain of the AGC amplifier is controlled using calculated average received power per predetermined time, the average received power cannot follow a change in power due to fast fading. Even when the gain of the AGC amplifier is controlled based on this average received power, the amplitudes of the I-component and Q-component baseband signals are not always made constant. Disadvantageously, an error occurs in the received signal.